I downloaded and listened to L.Mayer's recording, and noticed something interesting. During the Star Destroyer flyover, at 02:19, I can hear an audible "thump" which synchs up to that one explosion that didn't have an accompanying sound effect in the stereo and mono mixes, the one where they added that out-of-place ESB/ROTJ-era explosion sound in the '93 mix.

Could this sound be of the "missing" surround effects? Since the morgands1 recording doesn't have the surrounds, you can't hear it there. And since the '93 mix had that newer sound effect, we never heard this explosion effect. I always assumed it was added in to make up for the fact that the original mix didn't have a very audible sound effect there (up until now, all the pre-93 sources I've heard just have something that sounds like the echo of an explosion out in the distance), but now that I think about it, Burtt and co. did replace other sound effects in the '93 mix (like the blaster hit that scares Threepio into the escape pod), so we can't rule out that it was on the mix used as the base for '93, but replaced with the new effect.

(It wouldn't have been only in the baby boom channels, would it? I thought they just extracted the low frequencies from the other channels. And it sounds like it's in higher frequencies than just LFE...)

Of course, it could just be background noise or some glitch in the recording. Anybody wanna weigh in on this?

UPDATE: Listening to it again, I now think it's just the way the microphone picked up the percussion in the music, it occurs at one of the "DA DA DA DAAAA DAAA DAAA" stabs in the score. Thoughts?